Dibutyrl 3',5'-cyclic adenosine monophosphate (dibu cAMP) arrests rhythmic and synchronous contractions of interconnecting myocyte cultures prepared from fetal rat heart. Colchicine but not lumicochicine is able to reverse the inhibitory effect of dibu cAMP by reinitiating contractions. This proposal is an integrated multidisciplinary approach to the central question, "What are some of the basic structural characteristics of the heart cell which are important in cell contraction?" The specific aims are to characterize the effects of such agents as dibu cAMP, cAMP, theophiline, Ca ions, cytochalasin B and vinblastine sulfate on myocyte contraction. Since the opposing effects of the dibu cAMP and colchicine may implicate microtubules in myocyte contraction, electron microscopy will be utilized to determine any morphological changes before and after treatment with the various agents. Biochemical studies will be undertaken to isolate and assay for microtubules to help elucidate their role, if any, in myocyte contraction. In addition to the primary myocyte culture experiments, identical experiments will be repeated on an established striated muscle cell line (L6) and a heart muscle cell line (H9C2(2-1)). Enucleation studies will be used to determine if contractility occurs without the presence of a nucleus, and cell reconstruction experiments will be included to study nucleo-cytoplasmic interactions.